Photoimageable dielectrics in film form, rather than paste or liquid form, are desirable for a number of reasons. For example, liquid photoimageable dielectrics and paste photoimageable dielectrics run into and partially plug holes, such as via holes; upon development, they typically leave a residue in the hole. This leads to defective circuitized substrates and reduces yields. The film photoimageable dielectrics are also employed to cover xe2x80x9ctentxe2x80x9d holes, thereby eliminating hole filling operations which are expensive and inconvenient. However, for photoimageable dielectric films to remain flexible and not crack during fabrication of the film, application and processing, about a 12% solvent content is typically employed. The presence of the solvent, however, leads to additional problems. Often, such photoimageable dielectric films have a low Tg which in turn impairs dielectric properties of the photoimageable dielectric layer in the circuitized substrate. Also, such photoimageable dielectric films tend to stick to the photo-tool used in photoimaging. In an attempt to overcome these problems, such photoimageable dielectric films are baked prior to photoimaging, to drive off a portion of the solvent. However, the bake often causes the photoimageable dielectric film to become quite tacky while the photoimageable dielectric film is being heated. As a result, dust and debris from the bake oven and the ambient surroundings adheres to the surface of the photoimageable dielectric film.
During subsequent photoimaging of the photoimageable dielectric film, the dust and debris block the transmission of the ultraviolet light during the photoimaging step. As a result, during the subsequent development of the photoimageable dielectric film, the photoimageable dielectric film which was shielded from the UV by the dust and debris is unintentionally removed. The unintended removal of the photoimageable dielectric film produces an undesired void in the photoimaged dielectric film. When the workpiece is metallized, the void is often metallized, which may produce a short between circuitry lines.
It is desirable to have methods of fabricating circuitized structures which reduce the formation of unintentional voids or shorts, and which do not plug holes or leave residue in holes.
The present invention provides a new method for fabricating circuitized substrates which reduces the formation of shorts, and which does not require a bake step to drive off solvent before photoimaging. The method employs an essentially solventless photoimageable dielectric film, having a solvent content typically less than about 5%, preferably less than about 2%, and a glass transition temperature, when cured, which is greater than about 110xc2x0 C. The method for fabricating circuitized structures comprises the following steps: providing a photoimageable dielectric film, which film comprises: from about 95% to about 100% solids, comprising: from 0% to about 30% by weight of the solids, of a particulate rheology modifier; from about 70% to about 100% by weight of the solids of an epoxy resin system comprising: from about 85% to about 99.9% epoxy resins; and from about 0.1 to 15 parts by weight of the total resin weight, a cationic photoinitiator; from 0% to about 5% solvent; applying the photoimageable dielectric film to a circuitized substrate; then exposing the photoimageable dielectric film to actinic radiation.
The epoxy resin system comprises: from about 5% to about 80% of phenoxy polyol resin which is the condensation product of epichlorohydrin and bisphenol A, having a molecular weight of from about 40,000 to about 130,000; from about 0% to about 90% of an epoxidized multifunctional bisphenol A formaldehyde novolac resin having a molecular weight of from about 4,000 to about 10,000; from 20% to 80% of a diglycidyl ether of bisphenol A having a molecular weight of from about 600 to 2,500; from about 10% to about 35% liquid epoxy resin, having a molecular weight of from about 200 to about 600, preferably from about 250 to about 450. The xe2x80x9cliquid epoxy resinsxe2x80x9d are liquid at 20xc2x0 C. Preferably, the liquid epoxy resins are selected from the group consisting of: a cycloaliphatic epoxy resin, a bisphenol A epoxy resin, and mixtures thereof.
The solventless photoimageable dielectric film eliminates the need for a bake step before photoimaging; thus, the circuitized substrate with the non-crosslinked photoimageable dielectric film disposed thereon, is not exposed to, and does not accumulate dust and or debris from the bake oven.
The invention also relates to photoimageable dielectric films employed herein.